A pixel sensor comprises an array of pixel sensor cells that detects two dimensional signals. Pixel sensors include image sensors, which may convert a visual image to digital data that may be represented by a picture, i.e., an image frame. The pixel sensor cells are unit devices for the conversion of the two dimensional signals, which may be a visual image, into the digital data. A common type of pixel sensors includes image sensors employed in digital cameras and optical imaging devices. Such image sensors include charge-coupled devices (CCDs) or complementary metal oxide semiconductor (CMOS) image sensors.
While complementary metal oxide semiconductor (CMOS) image sensors have been more recently developed compared to the CCDs, CMOS image sensors provide an advantage of lower power consumption, smaller size, and faster data processing than CCDs as well as direct digital output that is not available in CCDs. Also, CMOS image sensors have lower manufacturing cost compared with the CCDs since many standard semiconductor manufacturing processes may be employed to manufacture CMOS image sensors. For these reasons, commercial employment of CMOS image sensors has been steadily increasing in recent years.
The image of an image sensor may be captured employing a “rolling shutter method.” In the rolling shutter method, the imaged is captured on a row-by-row basis within a pixel array, i.e., the image is captured contemporaneously for all pixels in a row, but the capture of the image is not contemporaneous between adjacent rows. Thus, the precise time of the image capture is the same only within a row, and is different from row to row. For highly dynamic subjects (such as objects moving at a high rate of speed), the rolling shutter methodology can create image artifacts.
To avoid such degradation of image quality, the image of an image sensor may be captured employing an “electronic global shutter method.” The global shutter method simulates a mechanical shutter using a global shutter operation, in which the image for the whole frame is captured at the almost exactly the same time for all the rows and columns. The signal, in photogenerated carriers, is then transferred to a corresponding floating diffusion node. The voltage at the floating diffusion nodes is read out of the imager array on a row-by-row basis. The global shutter method enables image capture of high speed subjects without image artifacts, but introduces a concern with the global shutter efficiency of the pixel since the integrity of the signal may be compromised by any charge leakage from the floating diffusion node between the time of the image capture and the time of the reading of the imager array.
In either operation mode, the image captured in the image sensor needs to be read out by a sensing circuitry that converts the charge generated in the photosensitive diode of each pixel to a digital output signal row by row. Typical image sensors have an image array size of 640×480 pixels, and array sizes exceeding 1024×1024 pixels are common in the industry. A typical readout time for a frame is from about 10 ms to 100 ms due to the large number of rows in the array size. Such a limitation on the readout time prevents capture of another image within the readout time, i.e., the image sensor may not capture another image during the readout time since the array of pixels must unload the data from the previously captured image frame before the image sensor can be ready to capture the next frame.
In view of the above, there exists a need for a CMOS image sensor with a “burst mode” operation capability, in which a plurality of successive images may be captured without interruption.
Further, the dynamic range of a CMOS image sensor pixel is finite. Capture of an image with a dynamic range exceeding the dynamic range of the image sensor pixels inevitably results in loss of information in pixels corresponding to extreme values of data. In addition, the extended dynamic range may be employed to reduce signal noise in the image.
Therefore, there exists a need for a CMOS image sensor with an extended dynamic range and/or reduced signal noise in the image, and methods for operating the same.